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    <title>Document</title>
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    <script>
        // 为数组原型prototype添加sum方法
        Array.prototype.sum = function () {
            var sum = 0;
            for (var i = 0; i < this.length; i++) {
                sum += this[i];
            }
            return sum;
        };
        var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
        // 数组就直接可以调用添加到原型上的sum方法了
        console.log(arr.sum())
    </script>
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